Sealed high-pressure arc lamp and socket therefor

ABSTRACT

A sealed beam arc lamp is disclosed wherein the lamp exterior is at a single electrical potential. The non-window portion of the lamp exterior can be of high strength metal alloy. Electrical insulation is provided by a thermally conductive ceramic cylinder internal to the lamp and disposed so as to avoid tensile stresses, and by a sealed labyrinthine passage defined by a removable socket.

United States Patent 1191 2,190,528 2/1940 Wright ..313/113 Parkman 51Feb. 6, 1973 541 SEALED HIGH-PRESSURE ARC LAM 3,502,929 3/1970 Richter..313/220 x AND SOCKET THEREFOR 3,549,934 12/1970 Peacher "313/1152,556,855 6/l95l St t ..3l3 204 X Inventor! l z r n Foster City,2,596,697 5/1952 .slaizzox Cai [73] Assignee: Varian Associates, PaloAlto, Calif. Primary Examiner Alfred Brody [22] F1 d M 13 1971Attorney-Stanley Z. Cole 1e ay 21 Appl. No.1 143,166 [57] ABSTRACT g Asealed beam arc lamp is disclosed wherein the lamp exterior is at asingle electrical potential. The non-win- [52] "31358 g 4 4 dow portionof the lamp exterior can be of high 2 l2 strength metal alloy.Electrical insulation is provided Cl. a thermally conductive ceramiccylinder internal to [-58] held of h-v-" the lamp and disposed so as toavoid tensile stresses, 313/2841 2851289 2881220, and by a sealedlabyrinthine passage defined by a removable socket. ['56] ReferencesCited UNITED STATES PATENTS 6 Claims 3 Drawing Figures SEALEDHIGH-PRESSURE ARC LAMP AND SOCKET THEREFOR BACKGROUND OF THE INVENTIONThis invention relates to gaseous discharge devices and in particular toan improved envelope and socket design for a sealed beam short arc lamp.The invention herein described was made in the course of or under astarted the ionization continues at low voltage.

In any lamp design various electrically conductive members will be atthe same potential as one or the other electrode. It is necessary,therefore, to provide sufficient insulation between the various memberpairs which are at different potential. This insulation must be disposedso as to prevent electrical breakdown between the electricallyconductive members when the starting voltage is applied except, asnoted, across the gap between the electrodes.

In prior art lamps, such as those described in U.S. Pat. No. 3,502,929issued Mar. 24, 1970, and U.S. Pat. applications Nos. 109,527 and 109,537, both filed Jan. 25, 1971, and assigned to the same assignee as thepresent invention, a ceramic cylinder was used which had a face as partof the lamp exterior. Such designs place major limitations on'lampoperation.

One limitation is that the ceramic cylinders are primarily under tensionduring operation because the high pressure in the lamp tends to push thelamp apart. Safe design stress limits for ceramics in tension have notbeen determined for uses of this type and the stress must therefore bekept below clearly safe levels.

A second majorlimitation arises because portions of the lamp exterioradjacent to the ceramic cylinder are 'at different potentials. If thelamp is operated in an atmosphere capable of carrying a current, e.g.,in the rain or under water, the lamp can easily arc across the ceramiccylinder, especially at the high starting voltage. Similarly it isdangerous to handle the lamp during start A third major limitationarises from the technique required to seal the ceramic insulator to themetal end members. The ceramic is commonly metallized and the metallizedportion of the ceramic is then brazed to the metalmembers with copper oran alloy that melts at reasonable brazing temperatures. Under the forcesimparted by the internal gas pressure, the brazed joint is either undertension or shear. Because of the very low creep strength of copper andthe commonly used brazing alloys, the allowable stress in the joint mustbe kept very low, resulting in a severely limited operating temperatureof the joint. This results in severe safety limitations for the lamp andlimits its use in many applications.

A fourth major limitation arises from the limitations on cooling thelamp. High strength ceramic must be used because of the previously notedstrength requirements. These ceramics have relatively low thermalconductivity, limiting the heat removal rate from the area of the lampcomprising the ceramic. High thermal conductivity ceramics such as BeOare noted'used because of their much lower strength. Most of the heat insuch lamps is removed by conduction through the metal member oppositethe window. Further, limitations are placed on the methods of heatdissipation. If a cooler is attached directly to the metal member, thenit must be carefully located in respect to the lamp holder so thatinsulation from the high voltage starting pulse is maintained. Also themetal surfaces carrying the high voltage are exposed to the ambientcooling air, an undesirable practice in many applications, e.g., whenair that is electrically conductive (e.g., from high moisture or saltcontent) must be used to cool the lamp. If the lamp is sealed in anenclosure to protect it from the electrically conductive ambient coolingair, then means must be provided to conduct the heat out of theenclosure. This results in a relatively long and inefficient heat path.

Another disadvantage of the prior art lamp is the requirement thatelectrical connections for the two electrodes be a substantial distanceapart.

Summaryof the Invention The present invention is an improved envelopeand socket design for a sealed beam short arc lamp. The ceramic cylinderprovided electrical insulation between the electrodes and a shortcooling path for one of the electrodes is internal to the lamp'andsocket. The exterior of the lamp is at a single potential and, exceptfor the window, can be completely metallic. A labyrinthine sealedpassage is formed when the lamp is inserted in its socket, preventingany breakdown between the electrical connections for the two electrodes.

The internal placement of the ceramic .insulators means that it will beprimarily under compressiveo'r shear stress during operation, which isdesirable for safest operation. The metallic exterior can betterwithstand tensile stress. It also provides advantages of superiorcooling ability, higher strength so as to enclose gas at higher pressureif desired, and higher temperatureoperating capability since allowablestress for alloys at elevated temperatures are well established by theASME Boiler Code. A single exterior potential makes it possible to startand operate the lamp in any atmosphere, including under water. It alsoallows a single socket to be used for the electrical connections to bothelectrodes.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1A and 1B are a top view andcross-sectional view, respectively, of an embodiment of the short arelamp of the present invention, without the socket.

FIG. 2 is a fragmentary, cross-sectional view of the embodiment of thearc lamp of FIG. I inserted into a socket of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIGS. 1A and 1B illustrate asealed beam short arc lamp of the present invention removed from thesocket providing electrical connections. One electrode, shown as cathode10, is mounted in the forward part of the lamp adjacent to window 11.The support for cathode is shown as the type described in detail andclaimed in copending U.S. application Ser..No. 143,377 filed May 14,1971, and assigned to the same assignee as the present invention.Cathode 10 is supported by struts 12 which are abutted to cathode 10 inslotted sleeve 13. The other ends of struts 12 are attached to ring 14at flaps (not shown) cut into lower portion 15. These flaps are detailedin the mentioned U.S. application Ser. No. 143,377. Reflector 16 issuspended from ring 14 and has slots 17 to accommodate struts 12. Theimportance of the various surfaces on ring 14 are described in detail incopending U.S. application Ser. No. 143,165 filed May 13, 197l,'andassigned to the same assignee as the present invention. Briefly thesesurfaces allow for accurate referencing of the tip of cathode 10 to thefocal point of reflector 16 and to the rest of the lamp.

The exterior of the forward portion of the lamp is cylindrical shell 18made of a high strength metallic alloy, such as stainless steel. Ring 14is attached to shell 18 at surface 19, cut into shell 18 andperpendicular to the axis of shell 18. This surface 19 controls thereferencing of the rest of the lamp to the tip of cathode 10," asmentioned above. Window 11 is attached to ring 20 by generallyU-shapedsealing ring 21. Ring 20 is attached to shell 18 to complete the forwardsubassembly. Rings such as ring 20 and sealing ring 21 are convenient touse to support window 11 because sealing ring 21 is under verylittlestress caused by the high pressure gas pushing against window 11. Thisstress is in window 11 itself and ring 20. Sealing ring 21 can thereforebe thin and can relieve the stress caused by the radial expansion ofwindow 11 as it heats up during assembly and operation of the lamp.

A second electrode, here an anode, is supported in the rear of the lamp.Here the anode assembly is in three parts: a forward cylindrical portion22 which is the tungsten anode anddefines the arc gap between theelectrodes, a base cylindrical portion 23 which provides surfaces forattachment to other parts of the lamp and from which heat is dissipated,as described below, and a rear cylindrical portion 24 to which electricpower is applied. The anode structure is supported on the axis ofceramic cylinder 25 by metallic ring 26 and heat transfer members 27.These members 27 are described in detail in copending U.S. Pat. No.application 135,472 filed Apr. 19, 1971 and assigned to.the sameassignee as the present invention. In the present embodiment members 27are of copper and are generally cone shaped. These members 27 deform inorder to relieve stresses caused by the differential expansion andcontraction of the tungsten anode portion 23, copper members 27 andceramic cylinders 25. Surrounding and attached to the ceramic cylinder25 is a high strength metal alloy shell 28, of a material havingapproximately the same thermal expansion coefficient as the ceramic,such as an alloy of iron, nickel and cobalt-sold. underthe trademarkKovar. Heat from the anode is in part dissipated out through ceramiccylinder '25 and-shell 28 to the lamp exterior where it can be removedby cooling means (not shown). A sealing ring 29 of, for example,'l(ovaris used tov seal metallic ring 26 to ceramic cylinder 25. This providesthe hermetic seal between the anode assembly and ceramic cylinder.

The connections on inner and outer faces of rings 27 need not behermetic but merely thermally conductive.

The importance of the backward extending portions of the anode structure24, of cylinder 25 and of shell 28, together with O-ring seat 31 arediscussed in connection with the socket shown in FIG. 2. The forwardextending portion of cylinder 25 helps to electrically insulate theanode structure from the lamp exterior.

The rear and front portions of the lamp are connected by attachingmetallic shell 28 to metallic shell 18. Air is evacuated and-the lamp isfilled with gas under pressure through pinch-off 30. With the lamp soassembled, in its socket, the lamp exterior, except for window 11, ismetallic.

It is recognized that shear stress may be resolved into tensile andcompressive components. It is the object of the invention to minimizethe tensile component because of the very high compressive strength ofceramics and thecomparatively much lower (and more unreliable) tensilestrength. It is the further object of the invention that the tensilestrength of ceramic available for use he used not purely to support theinternal gas pressure but to support primarily the thermal stresses thatarise when heat flows through the ceramic. When heat flows throughceramic, despite the high thermal conductivity of the ceramic used, adifference in temperature will arise between the area of entrance of theheat and the area of exit. Because of the thermal expansion of ceramicwith increased temperature and the relating non-ductile nature ofceramic, internal tensile stresses will arise because of the temperaturegradient.

28 and hoop'compressive force in ceramic 25. Then,

when the lamp is pressurized and operating, a greater capacity to absorbthe tensile component of the shear and thermal stresses results. Theforce on the anode assembly caused by the internal pressure is taken bythe ring 26 which imparts a compressive force on the end of the cylinder25. The force on the anode assembly and the cylinder 25 results in shearstresses in cylinder 25 near sleeve 28 and in the brazement, which, inthis embodiment, is the means of attaching cylinder 25 to sleeve 28.

Electric power for forward electrode 10 is provided at the back end ofshell 28, as described below, from where it passes through shell 18,ring 14' and struts 12 to reach forward electrode 10. This means thatthe lamp exterior is at a single potential and therefore the lamp can beoperated in an atmosphere capable of conducting electricity, which theprior art lamps could not.

FIG. 2 illustrates the improved socket of the present invention with thelamp inserted therein. The primary electrical contact for the forwardelectrode is provided by prongs 32. These can be of any convenient formmeeting the requirement of providing a good electric contact whenmetallic shell 28 is pushed into the socket. in the embodimentillustrated, prongs 32 have thin portion 34, allowing prongs 32 to actas springs so as to remain in contact with shell 28. Prongs 32 areattached to ring 33 to which electrical power will be applied asdescribed below. Metallic outer shell 35 of the socket overlaps metallicshell 28 of the lamp. Shell 35 is attached to ring 33 and is thereforeat the same potential, giving the lamp-socket combination a strongmetallic casing at a single potential. The two shells (28 and 35) arespaced apart by O-ring seal 36 in seat 31. This seal 36 allows theinterior of the socket to remain at atmospheric pressure and prevents anelectrically conductive atmosphere from entering the socket, asdiscussed below. The placement of the prongs 32 and O-ring 36 are amatter of convenience. The prongs could, for example, contact the insideof the sleeve 28 and the O-ring could be on the socket.

The electrical connection for the rear electrode is made to rear portion24 by metalllc flexible sleeve 37 attached to metallic cylinder 38.Sleeve 37 is of any convenient design providing a good electricalcontact when electrode portion 24 is inserted therein. Electrical powerwill be applied to cylinder 38, as described below.

A ceramic cylinder 42 separates the various members at the potential offorward electrode (e.g., ring 33, sleeve 28) from the members at thepotential of rear electrode 22 (e.g.,- cylinder 38, electrode portion24). If this cylinder 42 were of regular form so as to simply abut theend of ceramic cylinder 25 of the lamp, a surface, or worse a gap, couldbe present between these ceramic cylinders providing an easy path forelectrical discharge between members at different potential. Even if thecylinders 25 and 42 had precisely matching surfaces when the lamp andsocket were new, in use dirt can enter the socket or members can deformslightly, leaving a path for such a discharge. Therefore, a labyrinthinepassage 43 of sufficient length to prevent discharge is formed betweenmembers of different potential. One or more cylindrical extensions 41are formed in cylinder 42 with coMplementary cylindrical extensions 40formed in cylinder 25. Axial spacing is controlled by an external holder(not shown) attached to the socket and forward portion of the lamp, withpassage 43 and hole 39 in cylinder 38 allowing space for axial expansionof the various members. A total of three extensions. are shown but moreor less can be used to create the necessary path length. For example ata starting voltage of kv, three extensions are sufficient. Thelabyrinthine passage 43 is sealed from the lamp exterior by sealing ring36, as noted above, so that pressure can be maintained in the socketwhen the lamp is operated in an atmosphere of reduced pressure and tokeep out an electrically conductive atmosphere. Pressure must bemaintained because corona discharge occurs more easily at lowerpressures.

Electrical connections for both electrodesenter the lamp through tubes44 inserted through shell 35 into the base of the socket. Three equallyspaced tubes 44 are normally used for convenience in external mountingbut only two are used for the electrical connections, namely, wires 47and 48 attached to ring 33 (forward electrode) and cylinder 38 (rearelectrode), respectively. Wire 47 could be attached to the socketexterior but it is convenient to use an interior connection for greatersafety and reliability of electrical contact. Electrical insulation isprovided by filling the spaces 45 and 49 with an appropriate material,such as liquid porcelain. A cap 46 is placed over the end of the socketto protect the insulation.

While the socket has been described in terms of a removable lamp, it canof course be permanently attached. Then a hermetic seal could be used inplace of O-ring 36 and flexible connectors (sleeve 37 and prongs 32) arenot required. A single piece exterior shell could then be used in placeof shells 28 and 35 and a single rear electrode portion 24 could beused, eliminating separate cylinder 38. The same type of electricalconnection could be used although ring 33 could be eliminated.

Other types of electrical connections could be used with the presentinvention. For example, shell 35 and cylinder 38 could be, in effect,extended as far as desired in the form of a normal cable attachedthereto.

What is claimed is:

1. An arc lamp comprising:

a sealed envelope including a base and an Optical window opposite saidbase, said envelope containing an ionizable gas at a pressure higherthan atmospheric pressure,

the exterior of said lamp comprising a metallic portion, 7

a first electrode supported within said envelope 0pposite said base byconductive support means providing electrical contact between said firstelectrode and said metallic portion whereby the exterior of said lampcan be maintained at the same electrical potential as said firstelectrode,

a second electrode supported in said base, means for connecting thesecond electrode to a source of power,

a ceramic cylinder disposed between said second electrode and theexterior of said lamp so as to form at least part of the electricalinsulation of said second electrode from the exterior of said lamp,

and a reflector supported within said envelope.

2. An arc lamp as claimed in claim 1 wherein said ceramic cylindercomprises a heat dissipating material to transfer at least part of theheat from said base second electrode.

3. An arc lamp as claimed in claim 1 wherein the stress caused by saidhigh-pressure ionizable gas upon said ceramic cylinder is primarilycompressive.

4. An arc lamp as claimed in claim 1 wherein the base of said lamp isinsertable into an electric power providing socket which comprises saidmeans for connecting the second electrode to a source of power.

5. An arc lamp as claimed in claim 4 wherein ,the ceramic cylinder ofsaid lamp comprises one or more extenSions whereby when the base of saidlamp is inserted into a socket which also comprises a ceramic cylinderhaving one or more extensions, the extensions of said socket cylinderbeing interspersed with respect to the extensions of said lamp cylinder,a labyrinthine passage is created.

6. An arc lamp as claimed in claim 1 wherein the base of said lamp isirremovably attached and hermetically sealed to a member comprising aceramic cylinder having one or more extensions whereby a sealedlabyrinthine passage forming part of the electrical insulation of saidsecond electrode from said lamp exterior is created.

1. An arc lamp comprising: a sealed envelope including a base and anOptical window opposite said base, said envelope containing an ionizablegas at a pressure higher than atmospheric pressure, the exterior of saidlamp comprising a metallic portion, a first electrode supported withinsaid envelope opposite said base by conductive support means providingelectrical contact between said first electrode and said metallicportion whereby the exterior of said lamp can be maintained at the sameelectrical potential as said first electrode, a second electrodesupported in said base, means for connecting the second electrode to asource of power, a ceramic cylinder disposed between said secondelectrode and the exterior of said lamp so as to form at least part ofthe electrical insulation of said second electrode from the exterior ofsaid lamp, and a reflector supported within said envelope.
 1. An arclamp comprising: a sealed envelope including a base and an Opticalwindow opposite said base, said envelope containing an ionizable gas ata pressure higher than atmospheric pressure, the exterior of said lampcomprising a metallic portion, a first electrode supported within saidenvelope opposite said base by conductive support means providingelectrical contact between said first electrode and said metallicportion whereby the exterior of said lamp can be maintained at the sameelectrical potential as said first electrode, a second electrodesupported in said base, means for connecting the second electrode to asource of power, a ceramic cylinder disposed between said secondelectrode and the exterior of said lamp so as to form at least part ofthe electrical insulation of said second electrode from the exterior ofsaid lamp, and a reflector supported within said envelope.
 2. An arclamp as claimed in claim 1 wherein said ceramic cylinder comprises aheat dissipating material to transfer at least part of the heat fromsaid base second electrode.
 3. An arc lamp as claimed in claim 1 whereinthe stress caused by said high-pressure ionizable gas upon said ceramiccylinder is primarily compressive.
 4. An arc lamp as claimed in claim 1wherein the base of said lamp is insertable into an electric powerproviding socket which comprises said means for connecting the secondelectrode to a source of power.
 5. An arc lamp as claimed in claim 4wherein the ceramic cylinder of said lamp comprises one or moreextenSions whereby when the base of said lamp is inserted into a socketwhich also comprises a ceramic cylinder having one or more extensions,the extensions of said socket cylinder being interspersed with respectto the extensions of said lamp cylinder, a labyrinthine passage iscreated.